An optical fiber-based force transducer for microscale samples

Abstract

The present invention relates to an etched optical fiber as force transducer with feedback control, with a force range of 1-108 pN and a displacement range of 10-105 nm with a spatial resolution of the order of tens of nanometers are accessible with the instrument. The sample deformation (evolution) can be imaged simultaneously with rheological measurements. The device is used to perform extension rheology of polymer melts, silk, or other bio-fluids at microscales, measure mechanical responses of living cells such as neurons, muscle cells, etc. It is also used as a passive probe for microscale force measurements and to investigate properties of active suspensions such as bacterial baths.

We claim: 
     
         1 . Optical fiber based force transducer for microscale samples comprising:
 a laser( 1 ) which is placed above the objective of a microscope( 5 ),a piezoelectric transducer( 6 ) and an optical fiber ( 2 ) coupled to the said laser( 1 ), cantilever( 3 ) which is etched portion of the optical fiber senses force, a Position-Sensitive Detector ( 9 ) tracks the motion or deflection of the optical fiber cantilever( 3 ), a lamp( 13 ) within the microscope which illuminate the sample through a condenser( 7 ) using green light, a camera( 10 ) mounted on a side port of the microscope( 5 ) which records image, filters( 8 ) are placed in front of the camera( 10 ) and the Position-Sensitive Detector ( 9 ) which separates the green illumination light and the red laser light, computer interface ( 14 ) records Position-Sensitive Detector ( 9 ) and piezoelectric control( 11 ) reading, a custom-made software records and analyzes captured images in computer ( 14 ), sample( 4 ) is placed between cantilever( 3 ) and syringe needle( 15 ), the said syringe needle( 15 ) is connected to the piezoelectric transducer ( 6 ), linear polarizer and Fiber-optic connector ( 16 ) controls laser intensity and couples laser light to the said optical fiber ( 2 ), Quadrant Photodiode (QPD) mounted on a side port of the microscope, beam splitter( 12 ) to split laser light.   
     
     
         2 . A device as claimed in  claim 1  wherein, Quadrant Photodiode (QPD) measures force in the range of 10 −4 N to 10 −12 N. 
     
     
         3 . A device as claimed in  claim 1  wherein, the said device obtains a temporal resolution in the range of 10 −5  to 10 −7 s using a Quadrant Photodiode (QPD). 
     
     
         4 . A device as claimed in  claim 1  wherein, extension of the sample in the range of 10 −8  m to 10 −4  m is imposed with a spatial resolution in the range of 8 nm to 12 nm. 
     
     
         5 . A device as claimed in  claim 1  wherein, the said device allows independent control and measurement of the extensional strain or extensional force on a sample. 
     
     
         6 . A device as claimed in  claim 1  wherein, feedback-loop algorithm controls the extensional strain or extensional force on the sample. 
     
     
         7 . A device as claimed in  claim 1  wherein, operations are carried out in different modes like step-force, step-strain, or a specific force or strain protocol. 
     
     
         8 . A device as claimed in  claim 1  wherein, simultaneous video microscopy with sub-micron resolution is performed using phase-contrast, fluorescence or confocal modes of observation. 
     
     
         9 . A device as claimed in  claim 1  wherein, the rheological properties of samples are calculated for quantities in the range of 0.5 to 2 microlitres. 
     
     
         10 . A device as claimed in  claim 1  wherein, the said device maps extensional stresses within the sample, via birefringence. 
     
     
         11 . A device as claimed in  claim 1  wherein, samples includes living cells such as bacteria, muscle cells, neurons and biofluids such as blood, saliva, silkworm and spider silk.